The present invention relates to a structure of a disc wheel, installed on a vehicle, onto which tires are mounted, and more particularly to a structure of a disc wheel which improves the weld which joins a rim and a disc together.
Prior art discloses a rim and a disc of a disc wheel which are joined together by welding. The conventional disc wheel structure is disclosed in FIGS. 7, 8, 9(a), 9(b), 10(a), 10(b) and 10(c). FIG. 7 shows a cross-sectional view of a previously known disc wheel, while FIGS. 8, 9(a), 9(b), 10(a), 10(b) and 10(c) show enlarged views of the welded portion which joins the rim and disc together. FIGS. 7, 8, 9(a), 9(b), 10(a), 10(b) and 10(c) show a disc 2 made of casting or wrought products (including a material for forging) fitted to an inner surface 1a of a rim 1. The disc 2 comprises a main portion 2b extending radially outward and an angled, portion 2a which is in contact with and parallel to a portion 1a of a rim 1. Generally, arc welding is performed at a position between the surface 1a of rim 1 and the tip of the portion 2a of the disc 2, thereby forming a fillet weld thereon.
It is well known that a concentration of stress may occur in the rim 1 around the tip end 4 of the weld metal 3 when the angle .alpha., which is defined as the angle between the inner surface 1a of the rim 1 and the upper surface of weld metal 3, exceeds valves of 40.degree.. When .alpha. approaches or surpasses this 40.degree. valve, a crack "A", as shown in FIG. 8, may occur at a position where the tip of the weld metal 3 contacts the inner surface 1a of the rim 1. Therefore, to prevent such cracks from forming in the rim, it is necessary to maintain the angle .alpha. at a value less than 40.degree.. Even if an electrode wire 6 is oriented to maintain the angle .alpha. at an amount less than or equal to 40.degree., if an operator makes a mistake on the positioning of the wire 6, the resulting angle .alpha. may exceed the 40.degree. desired limit. If the wire 6 moves from its desired position, shown in FIG. 9(a), to the position indicated by the dotted line, shown in FIG. 9(b), the weld metal 3 envelopes the inner shoulder of portion 2a of the disc 2, and assumes a convex shape. As a result, the angle .alpha. may exceed the value of 40.degree., thereby resulting in stress concentration at the point 4.
A previous attempt to solve the problem of an erroneously placed electrode wire 6 was to decrease the amount of weld metal 3 used to join the rim to the disc. However, by reducing the amount of weld metal 3 used, as shown in FIG. 10(a), the thickness of the weld metal 3 correspondingly decreased at the tip end 4. Therefore, the strength of the joining weld subsequently decreases, and the risk of a crack occurring in the weld metal 3, as shown in FIG. 10(b), subsequently increases. Also, when the wire 6 is moved from its correct application position, the result may be the formation of an undercut C in the rim 1 and poor welding D at the top surface 2a of the disc 2, as shown in FIG. 10(c), thereby reducing the strength of the overall assembly. Therefore, it is not preferable to form an angle .alpha. which is less than or equal to 20.degree..
Consequently, in order to prevent cracks from forming in the rim 1 or the weld metal 3, it is preferable to maintain the angle .alpha. somewhere in the range between 20.degree. and 40.degree.. However, it is difficult to keep the electrode wire 6 in its correct application position, which subsequently results in the production of inferior disc wheel assemblies.